Coaxial see-through alignment imaging system

ABSTRACT

Aspects of the present disclosure provide an imaging system. For example, in the imaging system a first light source can generate a first light beam of a first wavelength, a second light source can generate a second light beam of a second wavelength, the second light beam having power sufficient to pass through at least a portion of a thickness of a wafer, an alignment module can coaxially align the second light beam with the first light beam, a coaxial module can focus the coaxially aligned first and second light beams onto a first pattern located on a front side of the wafer and a second pattern located below the first pattern, respectively, and an image capturing module can capture a first image of the first pattern and a second image of the second pattern. The second image can be captured via quantum tunneling imaging or infrared (IR) transmission imaging.

INCORPORATION BY REFERENCE

This present disclosure claims the benefit of U.S. ProvisionalApplication No. 63/066,779, “Method for Producing Overlay Results withAbsolute Reference for Semiconductor Manufacturing” filed on Aug. 17,2020, which is incorporated herein by reference in its entirety.

FIELD OF THE PRESENT DISCLOSURE

The present disclosure relates generally to methods of fabricatingsemiconductor devices and specifically to overlay error.

BACKGROUND

Semiconductor fabrication involves multiple varied steps and processes.One typical fabrication process is known as photolithography (alsocalled microlithography). Photolithography uses radiation, such asultraviolet or visible light, to generate fine patterns in asemiconductor device design. Many types of semiconductor devices, suchas diodes, transistors, and integrated circuits, can be constructedusing semiconductor fabrication techniques including photolithography,etching, film deposition, surface cleaning, metallization, and so forth.

SUMMARY

Aspects of the present disclosure also provide an imaging system. Forexample, the imaging system can include a first light source, a secondlight source, an alignment module, a coaxial module and an imagecapturing module. The first light source can be configured to generate afirst light beam of a first wavelength. The second light source can beconfigured to generate a second light beam of a second wavelength. Thealignment module can be configured to coaxially align the second lightbeam with the first light beam. The coaxial module can be configured tofocus the coaxially aligned first and second light beams onto a firstpattern located on a front side of the wafer and a second patternlocated below the first pattern, respectively. The image capturingmodule can be configured to capture a first image of the first patternand a second image of the second pattern. The second light beam can havepower sufficient to pass through at least a portion of a thickness ofthe wafer and reach the second pattern.

In an embodiment, the second wavelength can be longer than the firstwavelength. For example, the first light source can be a UV lightsource, and the second light source can be an IR light source, e.g.,including IR tunable quantum cascade lasers. As another example, thefirst wavelength can be 50-400 nanometers, e.g., 266 nanometers, and thesecond wavelength can be 1-10 micrometers, e.g., 3.6 or 3.7 micrometers.

In an embodiment, the image capturing module can capture the secondimage of the second pattern via quantum tunneling imaging or IRtransmission imaging.

In an embodiment, the second pattern can be incorporated in a referenceplate position below the wafer. In another embodiment, the imagingsystem can further include a substrate holder configured to hold thewafer, wherein the reference plate is incorporated in the substrateholder.

As can be appreciated, as fabrication progresses on a given wafer,depending on a given device being created, there can be many differentmaterials and layers. Thus each wafer at each process stage can have adifferent profile. This means a different wavelength may be needed topass through the wafer.

Of course, the order of discussion of the different steps as describedherein has been presented for clarity sake. In general, these steps canbe performed in any suitable order. Additionally, although each of thedifferent features, techniques, configurations, etc. herein may bediscussed in different places of this disclosure, it is intended thateach of the concepts can be executed independently of each other or incombination with each other. Accordingly, the present disclosure can beembodied and viewed in many different ways.

Note that this summary section does not specify every embodiment and/orincrementally novel aspect of the present disclosure or claimeddisclosure. Instead, this summary only provides a preliminary discussionof different embodiments and corresponding points of novelty overconventional techniques. For additional details and/or possibleperspectives of the present disclosure and embodiments, the reader isdirected to the Detailed Description section and corresponding figuresof the present disclosure as further discussed below.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of this disclosure that are proposed as exampleswill be described in detail with reference to the following figures,wherein like numerals reference like elements, and wherein:

FIG. 1A shows an industrial problem of overlay;

FIG. 1B shows overlay alleviation using an exemplary reference patternin accordance with some embodiments of the present disclosure;

FIG. 2 is a functional block diagram of an exemplary imaging system inaccordance with some embodiment of the present disclosure;

FIG. 3 is an enlarged view of a portion of coaxially aligned light beamsgenerated by the exemplary imaging system of FIG. 2 ;

FIG. 4A shows an enlarged top view of superimposed images of a portionof a wafer captured by the first and second image capturing devices ofthe exemplary imaging system of FIG. 2 in accordance with someembodiments of the present disclosure

FIG. 4B demonstrates exemplary image analysis for overlay calculationusing an absolute, independent reference pattern in accordance with someembodiments of the present disclosure; and

FIG. 5 is a flow chart illustrating an exemplary imaging method inaccordance with some embodiments of the present disclosure.

DETAILED DESCRIPTION

In accordance with the present disclosure, an imaging system and animaging method are provided, which use an absolute, independentreference pattern as an alignment mark for feature patterns to bealigned with. The feature patterns can be formed on a front side of awafer, and the reference pattern is independent of the front side of thewafer. For example, the reference pattern can be formed within or belowthe wafer. A first light beam (e.g., an ultraviolet (UV) light beam) ofa first wavelength can be used to image the feature patterns formed onthe first side of the wafer, and a second light beam (e.g., an infrared(IR) light beam) of a second wavelength can be used to image thereference pattern formed within or below the wafer. In an embodiment,the second light beam can be coaxially aligned with the first lightbeam. As the reference pattern is formed within or below the wafer, thesecond light beam has to “see through” a portion of the thickness or theentire thickness of the wafer in order to image the reference pattern.For example, the second light beam can have power or intensitysufficient to quantum tunnel through a portion of the thickness or theentire thickness of the wafer, depending on whether the referencepattern is formed within or below the wafer, to capture an image of thereference pattern using quantum tunneling imaging, IR transmissionimaging or the like. Therefore, UV images of the feature patterns and IRimages of the reference pattern can be captured in the same light axisand superimposed on each other. Image analysis can then be performed forexposure, inspection, alignment or other processing. Although the UV andIR images are captured coaxially, transmission to an image detector mayor may not be coaxial. For example, the coaxially captured images may beoptically separated and transmitted to separate image detectors asdiscussed below.

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.Further, spatially relative terms, such as “top,” “bottom,” “beneath,”“below,” “lower,” “above,” “upper” and the like, may be used herein forease of description to describe one element or feature's relationship toanother element(s) or feature(s) as illustrated in the figures. Thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

The order of discussion of the different steps as described herein hasbeen presented for clarity sake. In general, these steps can beperformed in any suitable order. Additionally, although each of thedifferent features, techniques, configurations, etc. herein may bediscussed in different places of this disclosure, it is intended thateach of the concepts can be executed independently of each other or incombination with each other. Accordingly, the present disclosure can beembodied and viewed in many different ways.

Microfabrication involves forming and processing multiple films andlayers on a wafer. This can include dozens or more films stacked on awafer. Patterns applied to the wafer for various films and layers needto be aligned to previously-formed patterns. Conventionally, suchalignment is realized by using part of the wafer to form alignment marksand scribe lines. However, the present inventors recognized that thevarious film deposition, etching, and treatment techniques at timescover the alignment marks and even completely remove the alignmentmarks. With alignment marks at times covered or missing, there can beerrors applying subsequent patterns on the wafer. The term overlay oroverlay error refers to the difference between placement of givenpattern relative to a previously-placed pattern. With alignment marksroutinely destroyed, overlay error can accumulate with additionallayers, which can cause poor performance and device error.

FIG. 1A illustrates an industrial problem of overlay. Each arrow hereinhas a starting point (e.g., 111A, 111B, 121A and 131A), whichcorresponds to a position of a preceding pattern, and an endpoint orarrowhead (e.g., 111A′, 111N′, 121N′), which corresponds to a positionof a subsequent pattern. As a result, each arrow represents an overlayvalue or overlay error when the subsequent pattern is formed over orside by side with the corresponding preceding pattern. In a process 110Afor example, there is no grid or reference plate when placing an initialpattern. Thus, a starting point 111A of a first arrow is likely to bemisaligned, that is, the initial pattern can have a placement error, forexample relative to the wafer edge. Then subsequent patterns try toalign based on the corresponding last pattern. As illustrated in FIB.1A, a starting point (e.g., 111B) of a subsequent arrow overlaps with anarrowhead (e.g., 111A′) of a corresponding last or preceding arrow. Notethat even in a theoretically perfect system, walkout can still occur.For example, if a system pattern placement tolerance is +/−4 nm and eachlevel references a previous level. Take a reference level to be 0 error.A first layer then could be +4 nm off. A second layer alignment to thefirst layer could be +4 nm off, meaning the second layer is now +8 nmoff the reference level. There are also process factors that induce orrelieve stress throughout fabrication that can induce walkout/alignmentshift even with pristine alignment marks visible that can add toaccumulated error.

Further, alignment marks may be destroyed at a step S120 in amanufacturing process, and placement again happens without a referencemark. Deterioration of alignment marks can cause accumulation ofalignment error with subsequent processing. Similar to the startingpoint 111A, a starting point 121A of a new arrow is likely to bemisaligned. In the example of FIG. 1A, the starting point 121A deviatesfrom an arrowhead 111N′. The process proceeds by aligning subsequentpatterns based on the corresponding last pattern until alignment marksare destroyed again at a step S130. Similarly, placement happens withouta reference mark, and a starting point 131A deviates from an arrowhead121N′. As can be seen in FIG. 1A, as layers increase, the overlay errorcan accumulate leading to poor manufacturing yield, device error, etc.Note that the process 110A is a non-limiting example. Other processes(e.g., 110B and 110C) may have different overlay values (differentarrows) and/or different steps.

With techniques herein, it is not necessary to create and re-createalignment marks. Instead, an absolute reference pattern, that is, areference pattern independent of the wafer or patterns stacked on thewafer. Each pattern applied to the wafer can be aligned to thisindependent reference pattern instead of being based off a previouspattern.

FIG. 1B shows overlay alleviation using an exemplary reference patternin accordance with some embodiments of the present disclosure. Withtechniques herein, all patterns (e.g., a pattern having a starting point141A) placed on a front side 191 of a wafer 190 are based on a samereference pattern 102. In an embodiment, the reference pattern 102 canbe located below the front side 191 of the wafer 190. For example, thereference pattern 102 can be formed on a back side 192 of the wafer 190or incorporated in the wafer 190. As another example, the referencepattern 102 can be incorporated in a reference plate (not shown in FIG.1B), and the reference plate can be positioned below the wafer 190,placed on or adhered to the back side 192 of the wafer 190, orincorporated in a substrate holder of a photolithography scanner orstepper (not shown in FIG. 1B) that is used to hold the wafer 190. Inother words, the reference pattern 102 is not affected by lithographicprocesses, such as etching, deposition, chemical mechanical polishingand the like, which are performed on the front side 191 of the wafer 190in order to form patterns. Therefore, the reference pattern 102 isindependent of the front side 191 of the wafer 190, and will be intactduring the lithographic processing of the wafer 190. Accordingly, thereference pattern 102 can be used and considered absolute, or rather,independent of any patterns formed on the front side 191 of the wafer190, and will not be changed from various deposition and etch stepsperformed on the wafer 190. In an embodiment, the reference pattern 102can be compared to the wafer 190 when placing a new pattern. For aninitial pattern, this means that the pattern can be fitted to thereference pattern 102. For subsequent patterns, this means that one ormore patterns can still be compared to the reference pattern 102 tocalculate overlay correction to return to a same alignment.

For example, in a process 140 the reference pattern 102 can be used toalign an initial pattern on the front side 191 of the wafer 190. In oneembodiment, the reference pattern 102 can be provided in a fixedposition relative to the wafer surface such as by embedding thereference pattern 102 within the wafer 190 or providing the referencepattern 102 fixed to a back side 192 of the wafer 190. Consequently, astarting point 141A of a first arrow is aligned to the reference pattern102, whose position is demonstrated as a reference line 150. Subsequentpatterns are also aligned using the fixed absolute, independentreference pattern 102. A new photoresist layer may be formed for eachsubsequent pattern, but no alignment marks need to be formed and/ordestroyed on the wafer 190 due to the reference pattern 102. As aresult, arrows center around the reference line 150, meaning that thesubsequent patterns are aligned to the reference pattern 102. Alignmentmay occur, for example, by moving a mask of the pattern image or movingthe wafer 190 relative to the mask. Overlay error is therefore unlikelyto accumulate as more and more layers are formed.

FIG. 2 is a functional block diagram of an exemplary imaging system 200in accordance with some embodiments of the present disclosure. Forexample, the exemplary imaging system 200 can be implemented in ascanner or a stepper of a lithography system. As another example, theexemplary imaging system 200 can be implemented in a resist coatingtool, e.g., CLEAN TRACK™ ACT™12 manufactured by Tokyo Electron Ltd, theresisting coating tool containing multiple mask-specific modules such asadvance softbake oven units, edge-bead removal modules, and cleaningsystems. The exemplary imaging system 200 can coaxially align two lightbeams of different wavelengths, focus the two coaxially aligned lightbeams onto a first pattern located on a front side of a substrate (e.g.,a wafer) and a second pattern located below the first pattern,respectively, and capture images of the first and second patterns. Forexample, the exemplary imaging system 200 can include a first lightsource 210, a second light source 220, an alignment module 230, acoaxial module 240, a first image capturing device 250 and a secondimage capturing device 260. The first image capturing device 250 and thesecond image capturing device 260 can be referred to as an imagecapturing module collectively.

In an embodiment, the first light source 210 can be configured togenerate a first incident light beam of a first wavelength. For example,the first light source 210 can be a UV light source that generates afirst incident light beam of 50-400 nanometers, e.g., 266 nanometers(shown in FIG. 2 as UV_(incident)). As another example, the first lightsource 210 can be an Optowaves (Optowares Inc., Massachusetts, USA)solid state lasers, such as pumped nanosecond laser for surface imaging.

In an embodiment, the second light source 220 can be configured togenerate a second incident light beam of a second wavelength. Accordingto some aspects of the present disclosure, as an absolute, independentreference pattern shall be located below a pattern that is to be formedon a front side of a wafer and the second incident light beam is used toimage the reference pattern, the second incident light beam has to seethrough at least a portion of a thickness or even the entire thicknessof the wafer, such as a wafer 290.

For example, the second incident light beam has power or intensitysufficient to pass through the entire thickness (e.g., 750 micrometers)of the wafer 290 to capture an image of the reference pattern usingquantum tunneling imaging, IR transmission imaging or the like. Asanother example, the second light source 220 can be an IR light sourcethat generates a second incident light beam of 1-10 micrometers, e.g.,3.6 or 3.7 micrometers (shown in FIG. 2 as IR_(incident)). In anembodiment, the second light source 220 can be IR tunable quantumcascade lasers, which can be obtained from Pranalytica, Inc.(California, USA). According to evanescent wave theory, a light beamimpinging at a surface (e.g., a front side 391 of the wafer 290, asshown in FIG. 3 ) between two different media (e.g., the wafer 290 andair or liquid in immersion lithography where the coaxial module 240 islocated) will have its intensity decayed exponentially perpendicular tothe surface. The penetration depth over which the intensity drops to 1/e(approximately 37%) depends on, among other things, the wavelength ofthe light beam. Typical penetration depth can be a fraction of thewavelength of the light beam, e.g., ⅕ of the wavelength, depending onthe incident angle of the light beam to the surface. As the secondwavelength of the second incident light beam IR_(incident) is muchlonger than the first wavelength of the first incident light beamUV_(incident), the second incident light beam IR_(incident), with powerwell controlled, can be capable of passing through the entire thicknessof the wafer 290.

In an embodiment, the relative position of the first (UV) light source210 and the second (IR) light source 220 can be calibrated periodically,which is also referred to as relative position of red and bluecalibration. For example, the relative position of the first lightsource 210 and the second light source 220 can be kept within a sensordynamic range which is a few decades and as such quite forgiving.Normalization, however, can be done with a stage artifact of knownrelative transmission being imaged as needed. For example, once a day sothat any relative intensity normalization can be conducted easily.Relative position or TIS tool induced shift calibrations are common tometrology stations. Relative position is recalibrated against the gridplate in real time as measurements are made. Accordingly, the exemplaryimaging system 200 can always have a real time absolute reference.Digital image capture and regression can be used.

In an embodiment, the alignment module 230 can be configured tocoaxially align the second incident light beam IR_(incident) with thefirst incident light beam UV_(incident). For example, the alignmentmodule 230 can include a first light beam splitter that splits the firstincident light beam UV_(incident) into two parts, one of which can betransmitted and the other of which can be reflected. In an embodiment,the first light beam splitter can be a prism. In another embodiment, thefirst light beam splitter can be a transparent plate, such as a sheet ofglass or plastic, coated on one side thereof with a partiallytransparent thin film of metal, such as aluminum, which allows one partof the first incident light beam UV_(incident) to be transmitted and theother part to be reflected. In the exemplary imaging system 200, thefirst light source 210 and the first light beam splitter can be arrangedsuch that the first incident light beam UV_(incident) is incident at a45-degree angle to the first light beam splitter.

For example, the alignment module 230 can further include a second lightbeam splitter that splits the second incident light beam IR_(incident)into two parts, one of which can be reflected and the other of which canbe transmitted. For example, the second light beam splitter can be aprism. As another example, the second light beam splitter can be a sheetof glass or plastic coated on one side thereof with a thin film ofaluminum, which allows one part of the second incident light beamIR_(incident) to be reflected and the other part to be transmitted. Inthe exemplary imaging system 200, the second light source 220 and thesecond light beam splitter can be arranged such that the second incidentlight beam IR_(incident) is incident at a 45-degree angle to the secondlight beam splitter.

For example, the alignment module 230 can further include a third beamsplitter that allows light beams of different wavelengths to be eitherreflected or transmitted. For example, the third beam splitter can be atransparent plate coated on one side thereof with a dichroic materialthat allows the first incident light beam UV_(incident) of the firstwavelength that is transmitted from the first light beam splitter to bereflected, and the second incident light beam IR_(incident) of thesecond wavelength that is transmitted from the second light beamsplitter to be transmitted. In an embodiment, the third beam splitter isdesigned and located such that the transmitted second incident lightbeam IR_(incident) is coaxially aligned with the reflected firstincident light beam UV_(incident) and the transmitted second incidentlight beam IR_(incident) and the reflected first incident light beamUV_(incident) can travel to the wafer 290 along the same light path.

In an embodiment, the coaxial module 240 can be configured to focus thefirst incident light beam UV_(incident) reflected from the third beamsplitter onto a first pattern 301 (shown in FIG. 3 ) located on a frontside 391 of the wafer 290, and focus the second incident light beamIR_(incident) transmitted from the third beam splitter onto a secondpattern 302 (or a reference pattern) located below the first pattern301. For example, the coaxial module 240 can be designed and configuredto adjust the tolerances of the placement (i.e., depth of focus (DOF))of the first pattern 301 and the second pattern 302. For example, alevel sensor can be used to track the top of the first pattern 301 andsubtract the height of the first pattern 301 by the height of the wafer290 to auto-adjust the DOFs of the coaxially aligned first incidentlight beam UV_(incident) and second incident light beam IR_(incident)simultaneously. With deep UV (DUV) light, photoresist damage can benegligible. A 250-micrometer field of view (FOV) herein corresponds withabout 60 nanometers per pixel in the case of 4K resolution. It issufficient for resolution of 0.1-nanometer registration errormeasurement. Having sufficient power or intensity of light source canmitigate any shadowing of metal layers. While FIG. 3 shows imaging of aphysical pattern formed in the wafer 290, images of a pattern to beformed (i.e., prior to exposure to activating light) may be realized bylight having a wavelength that does not activate photoresist in thewafer, for example.

In an embodiment, the coaxial module 240 can include 2-12 individualoptical elements, e.g., 6 optical elements. Each of the optical elementscan include sapphire, AN, MgF, CaF, BaF, LiF, Ge, Si, etc.

The first incident light beam UV_(incident) can be reflected by thefirst pattern 301 to form a first reflection light beam UV_(reflection).The first reflection light beam UV_(reflection) can be reflected by thethird beam splitter and the first light beam splitter sequentially andcaptured by the first image capturing device 250, and the first imagecapturing device 250 can form a corresponding first image of the firstpattern 301. For example, the first image capturing device 250 can beDataRay camera. The second incident light beam IR_(incident) can bereflected by the second pattern 302 to form a second reflection lightbeam IR_(reflection). The second reflection light beam IR_(reflection)can be transmitted by the third beam splitter and the second light beamsplitter sequentially and captured by the second image capturing device260, and the second image capturing device 260 can form a correspondingsecond image of the second pattern 302. For example, the second imagecapturing device 260 can be a high speed, high definition middlewavelength IR (MWIR) camera, e.g., FLIR X8500 MWIR. In an embodiment,image analysis can be performed on the first image and the second imageto calculate an overlay value to determine the placement of the firstpattern 301. For example, the image analysis can be accomplished bysuperimposing the first image of the first pattern 301 and the secondimage of the second pattern 302 on each other, and identifyingcoordinate locations of the first pattern 301 relative to the secondpattern 302. In some embodiments, the image analysis can be performed inreal time so that the placement of the first pattern 301 can be adjustedin real time.

In an embodiment, the alignment module 230 can further include a firstlens set and a second lens set. For example, the first lens set caninclude reflective and/or refractive optics that collimate the firstincident light beam UV_(incident) generated by the first light source210 and direct the collimated first incident light beam UV_(incident) tothe first light beam splitter. As another example, the second lens setcan also include reflective and/or refractive optics that collimate thesecond incident light beam IR_(incident) generated by the second lightsource 220 and direct the collimated second incident light beamIR_(incident) to the second light beam splitter.

In an embodiment, the exemplary imaging system 200 can further include athird lens set 270 and a fourth lens set 280. For example, the thirdlens set 270 can include reflective and/or refractive optics that focusthe first reflection light beam UV_(reflection) onto the first imagecapturing device 250. As another example, the fourth lens set 280 canalso include reflective and/or refractive optics that focus the secondreflection light beam IR_(reflection) onto the second image capturingdevice 260.

In an embodiment, the exemplary imaging system 200 can further includeoptics that can capture diffracted light beams outside of the coaxialmodule 240 and direct them to the first image capturing device 250 andthe second image capturing device 260.

In the exemplary embodiment shown in FIG. 3 , the first pattern 301 canbe included in a photomask (not shown) that is located on the front side391 of the wafer 290. In an embodiment, the photomask can be placed indirect contact with the wafer 290 in a contact printing system. Inanother embodiment, the photomask can be placed away from the wafer 290in a proximity printing system or in a projection printing system.

In the exemplary embodiment shown in FIG. 3 , the second pattern 302 islocated on a back side 392 of the wafer 290, and the second incidentlight beam IR_(incident) has power sufficient to pass through the entirethickness of the wafer 290 to capture the second image of the secondpattern 302 using quantum tunneling imaging, IR transmission imaging orthe like. In an embodiment, the second pattern 302 can be formed on areference plate 310. For example, the reference plate 310 can be a gridplate with 20 micrometers by 20 micrometers squares, nearly perfectlyaligned, and the second pattern 302 can be a corner point of at leastone of the squares. As another example, the reference plate 310 caninclude at least one of a point, a line, a corner, a box, a number, amark, or any other pattern that is suitable for alignment purpose, andthe second pattern 302 can be one of these. In an embodiment, thereference plate 310 can be adhered to the back side 392 of the wafer290. Accordingly, the reference plate 310 and the wafer 290 can functionas one module. In another embodiment, the reference plate 310 can beincorporated in a substrate holder 320 of a photolithography scanner orstepper. Although each time a given wafer may be placed on the substrateholder 320 in a different position or orientation as compared to aprevious placement, this does not matter. For a given new pattern to beplaced or exposed, the wafer can be imaged with the reference plate 310,e.g., the grid plate. The reference plate 310 can then provide arelatively reference point for identifying vectors to two or morepoints, from which vector analysis can be used to calculate an overlaycorrection adjustment in a next exposure. For example, when the wafer290, if having no pattern yet, is placed over the reference plate 310,the wafer 290 will be coarsely pre-aligned to the reference plate 310.As another example, when the wafer 290, if having an existing patternalready, is placed over the reference plate 310, the existing patternand the reference plate 310 can be co-axially aligned. In a conventionallithography process, measurement errors caused by wafer back sidescratches, back side dust and/or substrate distortion due to heat, mayimpact overlay, but conventional overlay systems are often blind tothese problems. Techniques herein include an independent reference plateand high spatial resolution to overcome these problems.

In an embodiment, the second pattern 302 can be formed on the back side392 of the wafer 290, and the second incident light beam IR_(incident)also has power sufficient to pass through the entire thickness of thewafer 290 to capture the second image of the second pattern 302 usingquantum tunneling imaging, IR transmission imaging or the like. Othertechniques can include embedding the second pattern 302 (e.g., gridlines) in the wafer 290 such as using a radioactive or fluorescentmaterial.

In an embodiment, the second pattern 302 can be formed on the front side291 of the wafer 290, and then a layer of silicon and/or silicon oxideis deposited thereon. For example, the layer of silicon and/or siliconoxide can have a thickness of 1-5 micrometers so that the second pattern302 is effectively “embedded” in the wafer 290 and patterns can beformed on the layer of silicon and/or silicon oxide. Accordingly, thesecond incident light beam IR_(incident) has to have power sufficient topass through the layer of silicon and/or silicon oxide in order tocapture the second image of the second pattern 302 using quantumtunneling imaging, IR transmission imaging or the like. As anotherexample, the second pattern 302 can be formed on the back side 292 ofthe wafer 290 before a protection layer, such as silicon or siliconoxide formed on the back side 292 of the wafer 290. Consequently, thesecond pattern 302 can also be embedded in the wafer 290. Accordingly,the second incident light beam IR_(incident) has to have powersufficient to pass through the entire thickness of the wafer 290 inorder to capture the second image of the second pattern 302 usingquantum tunneling imaging, IR transmission imaging or the like. In anembodiment, the second pattern 302 can be formed on a front side of acarrier wafer before the front side of the carrier wafer is bonded to aback side of a target wafer (e.g., the back side 392 of the wafer 290).As a result, the second pattern 302 can be sandwiched between thecarrier wafer and the target wafer, which together function as onewafer. Accordingly, the second incident light beam IR_(incident) has tohave power sufficient to pass through the entire thickness of the targetwafer in order to capture the second image of the second pattern 302using quantum tunneling imaging, IR transmission imaging or the like. Insome embodiments, light projection can also be used. For example, thesecond pattern 302 can be a projected grid that does not physicallyexist in the wafer 290, on a substrate holder or as a grid plate underthe substrate holder. In some embodiments, the second pattern 302 may bea combination of physical marks and light projection. For example,physical reference marks may be provided on a peripheral region of asubstrate holder that is not covered by a wafer placed on the substrateholder, and light projections can complete the reference pattern in thearea of the wafer such that tunneling may not be necessary.

FIG. 4A shows an enlarged top view of superimposed images of a portionof the wafer 290 captured by the first image capturing device 250 andthe second image capturing device 260, the portion including the firstpattern 301 and the second pattern 302, in accordance with someembodiments of the present disclosure. FIG. 4B demonstrates exemplaryimage analysis for overlay calculation using the first pattern 301,which acts as a reference pattern in an alignment process, in accordancewith some embodiments of the present disclosure. FIGS. 4A and 4B showhow the absolute, independent first pattern 301 can be used to calculatean overlay value of two patterns. This can be done by knowing eachcommon reference pattern to a co-ordinate system and using thatreference pattern to know “where” each pattern is in that co-ordinatesystem. Once that is known, for example the distance between each layer,the vector calculation required to extract the overlay value is donewith simple vector algebra. One can think of it as mix-match overlay(MMO) with a golden tool always there for oneself under the stage.

Each wafer may have scratch impact, heat impact and chucking issues,etc., which are deep enough to impact overlay. The wafer may furtherhave patterning defects, which may exist if lines are not connected asdesigned, critical dimensions are too small/too large, or there are gapsthat would cause shorts. As the second light beam can pass through thewafer 290, the second light beam can also see the defects, and thesecond image captured can further include the information of thedefects. In an embodiment, the image analysis can be performed on thefirst image of the first pattern 301 and the second image of the secondpattern 302 to inspect any defects of the wafer 290.

In an embodiment, the first pattern 301 (denoted by a point M), e.g.,the corner of one of the squares of the grid plate with 20 micrometersby 20 micrometers squares, can be considered absolute orwafer-independent and used to calculate an overlay value between thesecond pattern 302 (denoted by a point N) and a third pattern 401(denoted by a point P) that is formed subsequent to the formation of thesecond pattern 302. By superimposing the second pattern 302 on the firstpattern 301, a coordinate difference or vector {right arrow over (MN)}from the point M of the first pattern 301 to the point N of the secondpattern 302 can be determined. Likewise, by superimposing the thirdpattern 401 on the first pattern 301, another coordinate difference orvector {right arrow over (MP)} from the point M of the first pattern 301to the point P of the third pattern 401 can also be determined. Then, anoverlay value {right arrow over (NP)} between the point N and the pointP can be calculated: {right arrow over (NP)}={right arrow over(MP)}−{right arrow over (MN)}.

Further, with coordinate locations of points (e.g., N(Wx, Wy)) from thesecond pattern 302 and coordinate locations of points (e.g., P(Bx, By))from the third pattern 401, an overlay value or shift from the secondpattern 302 to the third pattern 401 can be determined. This overlayvalue can then be used to place the third or subsequent pattern tocorrect overlay relative to the independent reference pattern, e.g., thefirst pattern 301. In some embodiments, having a reference image that isuniform for every image comparison enables correcting adjacent patternsas well as keeping overlay corrections based on an initial line orabsolute reference. Regarding concerns about critical dimension (CD)variation effects for resist layers, extracting coordinates of thepatterns without pattern CD variation effects for resist layer andunder-layer (e.g., metal resist patterns cover most of via patterns). CDvariation effects for resist layer can be an issue for alignment and beignored by overlay measurement teams as negligible. Techniques hereinare far improved as the reference pattern itself is a far betterindication of pattern placement than an alignment mark that suffers fromCD's astigmatism and Zernike induced offset from patterns.

Note that in some embodiments, superimposing images is not necessary.Coordinate location data can be collected from the reference plate andthe working surface of the wafer, and then vector analysis can be usedto determine a gross offset or an overlay value.

FIG. 5 is a flow chart illustrating an exemplary imaging method 500 forprocessing a wafer (e.g., the wafer 290) in accordance with someembodiments of the present disclosure. The exemplary imaging method 500can be applied to the exemplary imaging system 200. In variousembodiments, some of the steps of the exemplary imaging method 500 showncan be performed concurrently or in a different order than shown, can besubstituted by other method steps, or can be omitted. Additional methodsteps can also be performed as desired.

At step S510, a first light beam of a first wavelength and a secondlight beam of a second wavelength can be generated. In an embodiment,the first light source 210 can be used to generate the first light beam,and the second light source 220 can be used to generate the second lightbeam. For example, the second wavelength can be longer than the firstwavelength. As another example, the first light source 210 can be a UVlight source, and the second light source 220 can be an IR light source.In an embodiment, the first wavelength is 50-400 nanometers, e.g., 266nanometers, and the second wavelength is 1-10 micrometers, e.g., 3.6 or3.7 micrometers.

At step S520, the first light beam can be coaxially aligned with thesecond light beam. In an embodiment, the alignment module 230 can beused to coaxially align the first light beam with the second light beam.For example, the first light beam splitter can be used to transmit thefirst light beam, the second light beam splitter can be used to reflectthe second light beam, and the third beam splitter can be used toreflect the first light beam transmitted from the first light beamsplitter, transmit the second light beam reflected from the second lightbeam splitter, and coaxially align the reflected first light beam withthe transmitted second light beam.

At step S530, the coaxially aligned first and second light beams can befocused onto a first pattern located on a front side of a wafer and asecond pattern located below the first pattern, respectively. In anembodiment, the coaxial module 240 can be used to focus the first lightbeam onto the first pattern and the second light beam onto the secondpattern. The second light beam can have power sufficient to pass throughat least a portion of the thickness of the wafer. In an embodiment, thesecond pattern can be incorporated in a reference plate, e.g., the gridplate with 20 micrometers by 20 micrometers squares with sub-nanometerspositional accuracy, located below the wafer. For example, the referenceplate can be placed on or adhered to the back side of the wafer.Accordingly, the second light beam can have power sufficient to passthrough the entire thickness of the wafer and reach the second pattern.In another embodiment, the reference plate can be incorporated in asubstrate holder or chick of a photolithography scanner or stepper, andthe exemplary imaging method 500 can further include a step of aligningthe reference plate with the wafer, prior to focusing the coaxiallyaligned first and second light beams. In yet another embodiment, thesecond pattern can be formed on the back side of the wafer. Accordingly,the second light beam can have power sufficient to pass through theentire thickness of the wafer and reach the second pattern. In stillanother embodiment, the second pattern can be embedded within the waferand accessible through one or more layers. Accordingly, the second lightbeam can have power sufficient to pass through a portion of thethickness of the wafer. For example, the second pattern can include aradioactive or fluorescent material. As another example, the secondpattern can include at least one of a point, a line, a corner, a box, atriangle, a number or a mark.

At step S540, a first image of the first pattern and a second image ofthe second pattern can be captured. For example, the first imagecapturing device 250 and the second image capturing device 260 can beused to capture the first image and the second image, respectively. Inan embodiment, the second image of the second pattern can be capturedvia quantum tunneling imaging or IR transmission imaging.

At step S550, image analysis can be performed on the first image and thesecond image to calculate an overlay value.

The exemplary imaging system 200 and the exemplary imaging method 500can be implemented as standalone coaxial metrology system and methodthat can operate in combination with a lithography tool, integratedtrack coaxial metrology system and method with feed forward to linkedlithography cells, or active coaxial metrology system and method thatcan be embedded in a lithography tool for real time correction.

Aspects of the present disclosure provide an imaging system and animaging method, which can provide an accurate and precise alignmentmechanism that does not rely on conventional alignment marks formed on afront surface of a wafer. Instead, with reference to a pattern or gridwithin/below the wafer, a reliable reference pattern can be repeatedlyaccessed for precise and accurate registration and alignment ofsubsequent patterns. The techniques herein will wipe out the need fortraditional overlay marks. These novel paradigms for overlay can requireno clear outs, no loss in real-estate and no complex scribe line design,making silicon area utilization improved and no complex integrations foralignment marks. The exemplary reference pattern disclosed herein willnot be impacted and wiped out by unfavorable processes that are makingdevices instead of the alignment marks, as they often areconventionally. Overlay placement accuracy can also now be measured fromthe very first layer where the second pattern is located, as thereference pattern is now not only near perfect every time but hiddenright under the stage there always.

In the preceding description, specific details have been set forth, suchas a particular geometry of a processing system and descriptions ofvarious components and processes used therein. It should be understood,however, that techniques herein may be practiced in other embodimentsthat depart from these specific details, and that such details are forpurposes of explanation and not limitation. Embodiments disclosed hereinhave been described with reference to the accompanying drawings.Similarly, for purposes of explanation, specific numbers, materials, andconfigurations have been set forth in order to provide a thoroughunderstanding. Nevertheless, embodiments may be practiced without suchspecific details. Components having substantially the same functionalconstructions are denoted by like reference characters, and thus anyredundant descriptions may be omitted.

Various techniques have been described as multiple discrete operationsto assist in understanding the various embodiments. The order ofdescription should not be construed as to imply that these operationsare necessarily order dependent. Indeed, these operations need not beperformed in the order of presentation. Operations described may beperformed in a different order than the described embodiment. Variousadditional operations may be performed and/or described operations maybe omitted in additional embodiments.

“Substrate” or “target substrate” as used herein generically refers toan object being processed in accordance with the present disclosure. Thesubstrate may include any material portion or structure of a device,particularly a semiconductor or other electronics device, and may, forexample, be a base substrate structure, such as a semiconductor wafer,reticle, or a layer on or overlying a base substrate structure such as athin film. Thus, substrate is not limited to any particular basestructure, underlying layer or overlying layer, patterned orun-patterned, but rather, is contemplated to include any such layer orbase structure, and any combination of layers and/or base structures.The description may reference particular types of substrates, but thisis for illustrative purposes only.

Those skilled in the art will also understand that there can be manyvariations made to the operations of the techniques explained abovewhile still achieving the same objectives of the present disclosure.Such variations are intended to be covered by the scope of thisdisclosure. As such, the foregoing descriptions of embodiments of thepresent disclosure are not intended to be limiting. Rather, anylimitations to embodiments of the present disclosure are presented inthe following claims.

What is claimed is:
 1. An imaging system, comprising: a first lightsource configured to generate a first light beam of a first wavelength;a second light source configured to generate a second light beam of asecond wavelength; an alignment module configured to coaxially align thesecond light beam with the first light beam; a coaxial module configuredto focus the coaxially aligned first and second light beams onto a firstpattern located on a front side of a wafer and a second pattern locatedbelow the first pattern, respectively; and an image capturing moduleconfigured to capture a first image of the first pattern and a secondimage of the second pattern, wherein the second light beam has powersufficient to pass through at least a portion of a thickness of thewafer and reach the second pattern.
 2. The imaging system of claim 1,wherein the second wavelength is longer than the first wavelength. 3.The imaging system of claim 2, wherein the first light source is anultraviolet (UV) light source, and the second light source is aninfrared (IR) light source.
 4. The imaging system of claim 3, whereinthe first wavelength is 50-400 nanometers, and the second wavelength is1-10 micrometers.
 5. The imaging system of claim 4, wherein the firstwavelength is 266 nanometers, and the second wavelength is 3.6 or 3.7micrometers.
 6. The imaging system of claim 3, wherein the IR lightsource includes IR tunable quantum cascade lasers.
 7. The imaging systemof claim 1, wherein an image capturing module captures the second imageof the second pattern via quantum tunneling imaging or IR transmissionimaging.
 8. The imaging system of claim 1, wherein the second pattern isincorporated in a reference plate positioned below the wafer.
 9. Theimaging system of claim 8, further comprising a substrate holderconfigured to hold the wafer, wherein the reference plate isincorporated in the substrate holder.